﻿using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;

namespace UtilLib
{
    /// <summary>
    /// 用于取得一个文件的编码方式(Encoding)
    /// </summary>
    public  class FileEncoding
    {
        /// <summary>
        /// 用于取得一个文本文件的编码方式(Encoding)
        /// </summary>
        public FileEncoding()
        {
        }

        /// <summary>
        /// 取得一个文本文件的编码方式,如果无法在文件头部找到有效的前导符,Encoding.Default将被返回
        /// 根据文件流的前4个字节判断Encoding
        /// Unicode {0xFF, 0xFE};
        /// BE-Unicode {0xFE, 0xFF};
        /// UTF8 = {0xEF, 0xBB, 0xBF};
        /// </summary>
        /// <param name="fileName">文件名</param>
        /// <returns></returns>
        public static Encoding GetEncoding(string fileName)
        {
            return GetEncoding(fileName, Encoding.Default);
        }

        /// <summary>
        /// 取得一个文本文件流的编码方式
        /// 根据文件流的前4个字节判断Encoding
        /// Unicode {0xFF, 0xFE};
        /// BE-Unicode {0xFE, 0xFF};
        /// UTF8 = {0xEF, 0xBB, 0xBF};
        /// </summary>
        /// <param name="stream">文本文件流</param>
        /// <returns></returns>
        public static Encoding GetEncoding(FileStream stream)
        {
            return GetEncoding(stream, Encoding.Default);
        }

        /// <summary>
        /// 取得一个文本文件的编码方式
        /// 根据文件流的前4个字节判断Encoding
        /// Unicode {0xFF, 0xFE};
        /// BE-Unicode {0xFE, 0xFF};
        /// UTF8 = {0xEF, 0xBB, 0xBF};
        /// </summary>
        /// <param name="fileName">文件名</param>
        /// <param name="defaultEncoding">默认编码方式,当该方法无法从文件的头部取得有效的前导符时,将返回该编码方式</param>
        /// <returns>Encoding</returns>
        public static Encoding GetEncoding(string fileName, Encoding defaultEncoding)
        {
            var fs = new FileStream(fileName, FileMode.Open);
            var targetEncoding = GetEncoding(fs, defaultEncoding);
            fs.Close();
            return targetEncoding;
        }
        /// <summary>
        /// 取得一个文本文件流的编码方式
        /// 根据文件流的前4个字节判断Encoding
        /// Unicode {0xFF, 0xFE};
        /// BE-Unicode {0xFE, 0xFF};
        /// UTF8 = {0xEF, 0xBB, 0xBF};
        /// </summary>
        /// <param name="stream">文本文件流</param>
        /// <param name="defaultEncoding">默认编码方式,当该方法无法从文件的头部取得有效的前导符时,将返回该编码方式</param>
        /// <returns>Encoding</returns>
        public static Encoding GetEncoding(FileStream stream, Encoding defaultEncoding)
        {
            Encoding targetEncoding = defaultEncoding;
            if (stream != null && stream.Length >= 2)
            {
                //保存文件流的前4个字节
                byte byte3 = 0;
                //保存当前Seek位置
                var origPos = stream.Seek(0, SeekOrigin.Begin);
                stream.Seek(0, SeekOrigin.Begin);
                var nByte = stream.ReadByte();
                var byte1 = Convert.ToByte(nByte);
                var byte2 = Convert.ToByte(stream.ReadByte());
                if (stream.Length >= 3)
                {
                    byte3 = Convert.ToByte(stream.ReadByte());
                }
                //if (stream.Length >= 4)
                //{
                //    Convert.ToByte(stream.ReadByte());
                //}
                //根据文件流的前4个字节判断Encoding
                //Unicode {0xFF, 0xFE};
                //BE-Unicode {0xFE, 0xFF};
                //UTF8 = {0xEF, 0xBB, 0xBF};
                if (byte1 == 0xFE && byte2 == 0xFF)//UnicodeBe
                {
                    targetEncoding = Encoding.BigEndianUnicode;
                }
                if (byte1 == 0xFF && byte2 == 0xFE && byte3 != 0xFF)//Unicode
                {
                    targetEncoding = Encoding.Unicode;
                }
                if (byte1 == 0xEF && byte2 == 0xBB && byte3 == 0xBF)//UTF8
                {
                    targetEncoding = Encoding.UTF8;
                }
                //恢复Seek位置
                stream.Seek(origPos, SeekOrigin.Begin);
            }
            return targetEncoding;
        }

    }
}
